[codegen 1.5] Implement ChannelToPortIoLoweringPass.

[copybara-service]

[codegen 1.5] Implement ChannelToPortIoLoweringPass.

This pass transforms channel operations (Send/Receive) within a ScheduledBlock into either block ports or FIFO instantiations.

We also add support for zero-latency, skid, and single-flop buffering on streaming I/O channels. We port helper functions from codegen 1.0 to add registers and the necessary control logic for Ready/Valid/Data interfaces, though we make it simpler by constructing skid buffers as the combination of a zero-latency & register buffer.

Features not yet supported:

• Gating received data (for predicated or non-blocking receives)